Light tube and power supply circuit

ABSTRACT

A replacement light tube for replacing a fluorescent light tube includes an elongate tubular housing having first and second ends, first and second end caps disposed thereon, each configured to fit with a socket for a fluorescent light tube, and a rigid support structure having a planar portion having a first surface extending within the elongate tubular housing between the first and second ends and having spaced-apart sidewalls extending away from the first surface and extending within the housing between the first and second ends. At least a portion of the sidewalls are in contact with an interior surface of housing. A plurality of light emitting diodes are supported only by a second surface of the planar portion opposite to the first surface and between the first and second ends, and are arranged to emit light through the housing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/777,331, filed Feb. 26, 2013, which is a continuation of U.S. patentapplication Ser. No. 12/965,019, filed Dec. 10, 2010 and issued as U.S.Pat. No. 8,382,327 on Feb. 26, 2013, which is a continuation of U.S.patent application Ser. No. 11/085,744, filed Mar. 21, 2005 and issuedas U.S. Pat. No. 8,247,985 on Aug. 21, 2012, which is a continuation ofU.S. patent application Ser. No. 09/782,375, filed Feb. 12, 2001 andissued as U.S. Pat. No. 7,049,761 on May 23, 2006, which claims thebenefit of U.S. Provisional Application No. 60/181,744 filed Feb. 11,2000.

FIELD OF THE INVENTION

The present invention relates to a light tube illuminated by LEDs (lightemitting diodes) which are packaged inside the light tube and powered bya power supply circuit.

BACKGROUND OF THE INVENTION

Conventional fluorescent lighting systems include fluorescent lighttubes and ballasts. Such lighting systems are used in a variety oflocations, such as buildings and transit buses, for a variety oflighting purposes, such as area lighting or backlighting. Althoughconventional fluorescent lighting systems have some advantages overknown lighting options, such as incandescent lighting systems,conventional fluorescent light tubes and ballasts have severalshortcomings. Conventional fluorescent light tubes have a short lifeexpectancy, are prone to fail when subjected to excessive vibration,consume high amounts of power, require a high operating voltage, andinclude several electrical connections which reduce reliability.Conventional ballasts are highly prone to fail when subjected toexcessive vibration. Accordingly, there is a desire to provide a lighttube and power supply circuit which overcome the shortcomings ofconventional fluorescent lighting systems. That is, there is a desire toprovide a light tube and power supply circuit which have a long lifeexpectancy, are resistant to vibration failure, consume low amounts ofpower, operate on a low voltage, and are highly reliable. It would alsobe desirable for such a light tube to mount within a conventionalfluorescent light tube socket.

SUMMARY OF THE INVENTION

Embodiments of a replacement light tube for replacing a fluorescentlight tube are disclosed herein. In one embodiment, the replacementlight tube includes an elongate tubular housing having a first end and asecond end and a first end cap and a second end cap disposed on thefirst end and the second end, respectively, each configured to fit witha socket for a fluorescent light tube. The replacement light tube alsoincludes a rigid support structure having a planar portion having afirst surface extending within the elongate tubular housing between thefirst end and the second end and having spaced-apart sidewalls extendingaway from the first surface and extending within the elongate tubularhousing between the first end and the second end. At least a portion ofthe sidewalls are in contact with an interior surface of the elongatetubular housing. Further, the replacement light tube includes aplurality of light emitting diodes supported only by a second surface ofthe planar portion opposite to the first surface and between the firstend and the second end. The plurality of light emitting diodes arearranged to emit light through the elongate tubular housing.

In another embodiment, the replacement light tube includes an elongatetubular housing having a first end and a second end and a first end capand a second end cap disposed on the first end and the second end,respectively, each configured to fit with a socket for a fluorescentlight tube and comprising a respective pair of electrical bi-pinconnectors. The replacement light tube also includes a rigid supportstructure having a planar portion having a first surface extendingwithin the elongate tubular housing between the first end and the secondend and having spaced-apart sidewalls extending away from the firstsurface and extending within the elongate tubular housing between thefirst end and the second end. At least a portion of the sidewalls are incontact with an interior surface of the elongate tubular housing.Further, the replacement light tube includes a plurality of lightemitting diodes supported only by a second surface of the planar portionopposite to the first surface and between the first end and the secondend. The plurality of light emitting diodes are arranged to emit lightthrough the elongate tubular housing. The support structure divides theelongate tubular housing into a first space in which the plurality oflight emitting diodes are housed and a second space defined by theplanar portion, the sidewalls and the interior surface of the elongatetubular housing.

In another embodiment, the replacement light tube includes an elongatetubular housing having a first end and a second end, and a first end capand a second end cap disposed on the first end and the second end,respectively, each configured to fit with a socket for a fluorescentlight tube. The replacement light tube also includes a rigid supportstructure having a planar portion having a first surface extendingwithin the elongate tubular housing between the first end and the secondend and having spaced-apart sidewalls extending away from the firstsurface and extending within the elongate tubular housing between thefirst end and the second end. At least a portion of the sidewalls are incontact with an interior surface of the elongate tubular housing.Further, the replacement light tube includes a plurality of lightemitting diodes supported by the first surface between the first end andthe second end. The plurality of light emitting diodes are arranged toemit light through the elongate tubular housing. The replacement lighttube further includes a power supply circuit including a pulse widthmodulator and a current limiter. The power supply circuit is packagedwithin one of the end caps.

In another embodiment, the replacement light tube includes an elongatetubular housing having a first end and a second end and a first end capand a second end cap disposed on the first end and the second end,respectively, each configured to fit with a socket for a fluorescentlight tube. The replacement light tube also includes a rigid supportstructure having a planar portion having a first surface extendingwithin the elongate tubular housing between the first end and the secondend and having spaced-apart sidewalls extending away from the planarportion and extending within the elongate tubular housing between thefirst end and the second end. At least a portion of the sidewalls are incontact with an interior surface of the elongate tubular housing. Theplanar portion is integral with the sidewalls. Further, the replacementlight tube includes a plurality of light emitting diodes supported bythe first surface between the first end and the second end. Theplurality of light emitting diodes are arranged to emit light throughthe elongate tubular housing. The replacement light tube furtherincludes a power supply circuit including a pulse width modulator and acurrent limiter. At least a portion of the power supply circuit ispackaged within the elongate tubular housing or one of the end caps.

These and other embodiments will be discussed in additional detailhereafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The description herein makes reference to the accompanying drawingswherein like reference numerals refer to like parts throughout theseveral views, and wherein:

FIG. 1 is a line drawing showing a light tube, in perspective view,which in accordance with the present invention is illuminated by LEDspackaged inside the light tube;

FIG. 2 is a perspective view of the LEDs mounted on a circuit board;

FIG. 3 is a cross-sectional view of FIG. 2 taken along lines 3-3;

FIG. 4 is a fragmentary, perspective view of one embodiment of thepresent invention showing one end of the light tube disconnected fromone end of a light tube socket;

FIG. 5 is an electrical block diagram of a first power supply circuitfor supplying power to the light tube;

FIG. 6 is an electrical schematic of a switching power supply typecurrent limiter;

FIG. 7 is an electrical block diagram of a second power supply circuitfor supplying power to the light tube;

FIG. 8 is an electrical block diagram of a third power supply circuitfor supplying power to the light tube;

FIG. 9 is a fragmentary, perspective view of another embodiment of thepresent invention showing one end of the light tube disconnected fromone end of the light tube socket; and

FIG. 10 is an electrical block diagram of a fourth power supply circuitfor supplying power to the light tube.

DETAILED DESCRIPTION

FIG. 1 is a line drawing showing a light tube 20 in perspective view. Inaccordance with the present invention, the light tube 20 is illuminatedby LEDs 22 packaged inside the light tube 20. The light tube 20 includesa cylindrically shaped bulb portion 24 having a pair of end caps 26 and28 disposed at opposite ends of the bulb portion. Preferably, the bulbportion 24 is made from a transparent or translucent material such asglass, plastic, or the like. As such, the bulb material may be eitherclear or frosted.

In a preferred embodiment of the present invention, the light tube 20has the same dimensions and end caps 26 and 28 (e.g. electrical malebi-pin connectors, type G13) as a conventional fluorescent light tube.As such, the present invention can be mounted in a conventionalfluorescent light tube socket.

The line drawing of FIG. 1 also reveals the internal components of thelight tube 20. The light tube 20 further includes a circuit board 30with the LEDs 22 mounted thereon. The circuit board 30 and LEDs 22 areenclosed inside the bulb portion 24 and the end caps 26 and 28.

FIG. 2 is a perspective view of the LEDs 22 mounted on the circuit board30. A group of LEDs 22, as shown in FIG. 2, is commonly referred to as abank or array of LEDs. Within the scope of the present invention, thelight tube 20 may include one or more banks or arrays of LEDs 22 mountedon one or more circuit boards 30. In a preferred embodiment of thepresent invention, the LEDs 22 emit white light and, thus, are commonlyreferred to in the art as white LEDs. In FIGS. 1 and 2, the LEDs 22 aremounted to one surface 32 of the circuit board 30. In a preferredembodiment of the present invention, the LEDs 22 are arranged to emit orshine white light through only one side of the bulb portion 24, thusdirecting the white light to a predetermined point of use. Thisarrangement reduces light losses due to imperfect reflection in aconvention lighting fixture. In alternative embodiments of the presentinvention, LEDs 22 may also be mounted, in any combination, to the othersurfaces 34, 36, and/or 38 of the circuit board 30.

FIG. 3 is a cross-sectional view of FIG. 2 taken along lines 3-3. Toprovide structural strength along the length of the light tube 20, thecircuit board 30 is designed with a H-shaped cross-section. To produce apredetermined radiation pattern or dispersion of light from the lighttube 20, each LED 22 is mounted at an angle relative to adjacent LEDsand/or the mounting surface 32. The total radiation pattern of lightfrom the light tube 20 is effected by (1) the mounting angle of the LEDs22 and (2) the radiation pattern of light from each LED. Currently,white LEDs having a viewing range between 6° and 45° are commerciallyavailable.

FIG. 4 is a fragmentary, perspective view of one embodiment of thepresent invention showing one end of the light tube 20 disconnected fromone end of a light tube socket 40. Similar to conventional fluorescentlighting systems and in this embodiment of the present invention, thelight tube socket 40 includes a pair of electrical female connectors 42and the light tube 20 includes a pair of mating electrical maleconnectors 44.

Within the scope of the present invention, the light tube 20 may bepowered by one of four power supply circuits 100, 200, 300, and 400. Afirst power supply circuit includes a power source and a conventionalfluorescent ballast. A second power supply circuit includes a powersource and a rectifier/filter circuit. A third power supply circuitincludes a DC power source and a PWM (Pulse Width Modulation) circuit. Afourth power supply circuit powers the light tube 20 inductively.

FIG. 5 is an electrical block diagram of a first power supply circuit100 for supplying power to the light tube 20. The first power supplycircuit 100 is particularly adapted to operate within an existing,conventional fluorescent lighting system. As such, the first powersupply circuit 100 includes a conventional fluorescent light tube socket40 having two electrical female connectors 42 disposed at opposite endsof the socket. Accordingly, a light tube 20 particularly adapted for usewith the first power supply circuit 100 includes two end caps 26 and 28,each end cap having the form of an electrical male connector 44 whichmates with a corresponding electrical female connector 42 in the socket40.

The first power supply circuit 100 also includes a power source 46 and aconventional magnetic or electronic fluorescent ballast 48. The powersource 46 supplies power to the conventional fluorescent ballast 48.

The first power supply circuit 100 further includes a rectifier/filtercircuit 50, a PWM circuit 52, and one or more current-limiting circuits54. The rectifier/filter circuit 50, the PWM circuit 52, and the one ormore current-limiting circuits 54 of the first power supply circuit 100are packaged inside one of the two end caps 26 or 28 of the light tube20.

The rectifier/filter circuit 50 receives AC power from the ballast 48and converts the AC power to DC power. The PWM circuit 52 receives theDC power from the rectifier/filter circuit 50 and pulse-width modulatesthe DC power to the one or more current-limiting circuits 54. In apreferred embodiment of the present invention, the PWM circuit 52receives the DC power from the rectifier/filter circuit 50 andcyclically switches the DC power on and off to the one or morecurrent-limiting circuits 54. The DC power is switched on and off by thePWM circuit 52 at a frequency which causes the white light emitted fromthe LEDs 22 to appear, when viewed with a “naked” human eye, to shinecontinuously. The PWM duty cycle can be adjusted or varied by controlcircuitry (not shown) to maintain the power consumption of the LEDs 22at safe levels.

The DC power is modulated for several reasons. First, the DC power ismodulated to adjust the brightness or intensity of the white lightemitted from the LEDs 22 and, in turn, adjust the brightness orintensity of the white light emitted from the light tube 20. Optionally,the brightness or intensity of the white light emitted from the lighttube 20 may be adjusted by a user. Second, the DC power is modulated toimprove the illumination efficiency of the light tube 20 by capitalizingupon a phenomenon in which short pulses of light at high brightness orintensity to appear brighter than a continuous, lower brightness orintensity of light having the same average power. Third, the DC power ismodulated to regulate the intensity of light emitted from the light tube20 to compensate for supply voltage fluctuations, ambient temperaturechanges, and other such factors that affect the intensity of white lightemitted by the LEDs 22. Fourth, the DC power is modulated to raise thevariations of the frequency of light above the nominal variation of 120to 100 Hz thereby reducing illumination artifacts caused by lowfrequency light variations, including interactions with video screens.Fifth, the DC power may optionally be modulated to provide an alarmfunction wherein light from the light tube 20 cyclically flashes on andoff.

The one or more current-limiting circuits 54 receive the pulse-widthmodulated or switched DC power from the PWM circuit 52 and transmit aregulated amount of power to one or more arrays of LEDs 22. Eachcurrent-limiting circuit 54 powers a bank of one or more white LEDs 22.If a bank of LEDs 22 consists of more than one LED, the LEDs areelectrically connected in series in an anode to cathode arrangement. Ifbrightness or intensity variation between the LEDs 22 can be tolerated,the LEDs can be electrically connected in parallel.

The one or more current-limiting circuits 54 may include (1) a resistor,(2) a current-limiting semiconductor circuit, or (3) a switching powersupply type current limiter.

FIG. 6 is an electrical schematic of a switching power supply typecurrent limiter 56. The limiter 56 includes an inductor 58, electricallyconnected in series between the PWM circuit 52 and the array of LEDs 22,and a power diode 60, electrically connected between ground 62 and a PWMcircuit/inductor node 64. The diode 60 is designed to begin conductionafter the PWM circuit 52 is switched off. In this case, the value of theinductor 58 is adjusted in conjunction with the PWM duty cycle toprovide the benefits described above. The switching power supply typecurrent limiter 56 provides higher power efficiency than the other typesof current-limiting circuits listed above.

FIG. 7 is an electrical block diagram of a second power supply circuit200 for supplying power to the light tube 20. Similar to the first powersupply circuit 100, the second power supply circuit 200 includes aconventional fluorescent light tube socket 40 having two electricalfemale connectors 42 disposed at opposite ends of the socket 40.Accordingly, a light tube 20 particularly adapted for use with thesecond power supply circuit 200 includes two end caps 26 and 28, eachend cap having the form of an electrical male connector 44 which mateswith a corresponding electrical female connector 42 in the socket 40.

In the second power supply circuit 200, the power source 46 suppliespower directly to the rectifier/filter circuit 50. The rectifier/filtercircuit 50, the PWM circuit 52, and the one or more current-limitingcircuits 54 operate as described above to power the one or more arraysof LEDs 22. The rectifier/filter circuit 50, the PWM circuit 52, and theone or more current-limiting circuits 54 of the second power supplycircuit 200 are preferably packaged inside the end caps 26 and 28 or thebulb portion 24 of the light tube 20 or inside the light tube socket 40.

FIG. 8 is an electrical block diagram of a third power supply circuit300 for supplying power to the light tube 20. Similar to the first andsecond power supply circuits 100 and 200, the third power supply circuit300 includes a conventional fluorescent light tube socket 40 having twoelectrical female connectors 42 disposed at opposite ends of the socket40. Accordingly, a light tube 20 particularly adapted for use with thethird power supply circuit 300 includes two end caps 26 and 28, each endcap having the form of an electrical male connector 44 which mates witha corresponding electrical female connector 42 in the socket 40.

The third power supply circuit 300 includes a DC power source 66, suchas a vehicle battery. In the third power supply circuit 300, the DCpower source 66 supplies DC power directly to the PWM circuit 52. ThePWM circuit 52 and the one or more current-limiting circuits 54 operateas described above to power the one or more arrays of LEDs 22. In thethird power supply circuit 300, the PWM circuit 52 is preferablypackaged in physical location typically occupied by the ballast of aconventional fluorescent lighting system while the one or morecurrent-limiting circuits 54 and LEDs 22 are preferably packaged insidethe light tube 20, in either one of the two end caps 26 or 28 or thebulb portion 24.

FIG. 9 is a fragmentary, perspective view of another embodiment of thepresent invention showing one end of the light tube 20 disconnected fromone end of the light tube socket 40. In this embodiment of the presentinvention, the light tube socket 40 includes a pair of brackets 68 andthe light tube 20 includes a pair of end caps 26 and 28 which mate withthe brackets 68.

FIG. 10 is an electrical block diagram of a fourth power supply circuit400 for supplying power to the light tube 20. Unlike the first, second,and third power supply circuits 100, 200, and 300 which are poweredthrough direct electrical male and female connectors 44 and 42, thefourth power supply circuit 400 is powered inductively. As such, thefourth power supply circuit 400 includes a light tube socket 40 havingtwo brackets 68 disposed at opposite ends of the socket 40. At least onebracket 68 includes an inductive transmitter 70. Accordingly, a lighttube 20 particularly adapted for use with the fourth power supplycircuit 400 has two end caps 26 and 28 with at least one end capincluding an inductive receiver or antenna 72. When the light tube 20 ismounted in the light tube socket 40, the at least one inductive receiver72 in the light tube 20 is disposed adjacent to the at least oneinductive transmitter 70 in the light tube socket 40.

The fourth power supply circuit 400 includes the power source 46 whichsupplies power to the at least one inductive transmitter 70 in the lighttube socket 40. The at least one transmitter 70 inductively suppliespower to the at least one receiver 72 in one of the end caps 26 and/or28 of the light tube 20. The at least one inductive receiver 72 suppliespower to the rectifier/filter circuit 50. The rectifier/filter circuit50, PWM circuit 52, and the one or more current-limiting circuits 54operate as described above to power the one or more arrays of LEDs 22.In this manner, the light tube 20 is powered without direct electricalconnection.

What is claimed is:
 1. A replacement light tube for replacing afluorescent light tube, comprising: an elongate tubular housing having afirst end and a second end; a first end cap and a second end capdisposed on the first end and the second end, respectively, eachconfigured to fit with a socket for the fluorescent light tube; a rigidsupport structure enclosed within the elongate tubular housing andhaving a planar portion having a first surface extending within theelongate tubular housing between the first end and the second end, therigid support structure having spaced-apart sidewalls extending awayfrom the first surface and extending within the elongate tubular housingbetween the first end and the second end, at least a portion of thesidewalls in contact with an interior surface of the elongate tubularhousing; a plurality of light emitting diodes supported by a secondsurface of the planar portion opposite to the first surface and betweenthe first end and the second end, the plurality of light emitting diodesarranged to emit light through the elongate tubular housing; and a powersupply circuit including a pulse width modulator and at least twocurrent limiting elements, the at least two current limiting elementsconfigured to receive power from the pulse width modulator and totransmit power to at least some of the plurality of light emittingdiodes.
 2. The replacement light tube of claim 1 wherein the sidewallseach have a substantially uniform cross-section along a length of theelongate tubular housing.
 3. The replacement light tube of claim 1wherein the rigid support structure has a substantially uniformcross-section along a length of the elongate tubular housing.
 4. Thereplacement light tube of claim 1 wherein the first surface extends in aplane within the elongate tubular housing and wherein the sidewallscontact the interior surface of the elongate tubular housing away fromthe plane in which the first surface extends.
 5. The replacement lighttube of claim 1 wherein the planar portion, the sidewalls and theinterior surface of the elongate tubular housing form a space extendingbetween the first end and the second end.
 6. The replacement light tubeof claim 1 wherein the support structure divides the elongate tubularhousing into a first space in which the plurality of light emittingdiodes are housed and a second space defined by the planar portion, thesidewalls and the interior surface of the elongate tubular housing. 7.The replacement light tube of claim 1 wherein the planar portion isintegral with the sidewalls.
 8. The replacement light tube of claim 1,wherein at least a portion of the power supply circuit is packagedinside the elongate tubular housing.
 9. The replacement light tube ofclaim 1 wherein the power supply circuit includes a rectifier configuredto receive alternating current (AC) input and to provide direct current(DC) output.
 10. The replacement light tube of claim 9 wherein therectifier is arranged to provide the DC output to the pulse widthmodulator.
 11. The replacement light tube of claim 8 wherein the pulsewidth modulator switches direct current (DC) power to the plurality oflight emitting diodes on and off at a frequency such that the lightemitted from the plurality of light emitting diodes appears, when viewedby a human eye, to shine continuously.
 12. The replacement light tube ofclaim 1 wherein at least one of the at least two current limitingelements comprises an inductive element electrically coupled between thepulse width modulator and the at least some of the plurality of lightemitting diodes.
 13. The replacement light tube of claim 1, wherein theat least two current limiting elements and the pulse width modulator arepackaged outside of the elongate tubular housing.
 14. The replacementlight tube of claim 1 wherein a switching power supply type currentlimiter includes at least one of the current limiting elements.
 15. Thereplacement light tube of claim 1 wherein the plurality of lightemitting diodes is arranged in spaced-apart banks of light emittingdiodes such that a spacing between light emitting diodes in each bank isless than a spacing between centers of adjacent banks, each of thespaced-apart banks including more than one light emitting diode.
 16. Thereplacement light tube of claim 1 wherein the first end cap and thesecond end cap comprise a respective pair of electrical bi-pinconnectors.
 17. The replacement light tube of claim 1 wherein the lightemitting diodes are arranged as a plurality of groups spaced along anaxis extending between the first end and second end, each groupcomprising light emitting diodes spatially displaced from each other ina direction orthogonal to the axis.
 18. A replacement light tube forreplacing a fluorescent light tube, comprising: an elongate tubularhousing having a first end and a second end; a first end cap and asecond end cap disposed on the first end and the second end,respectively, each configured to fit with a socket for the fluorescentlight tube and comprising a respective pair of electrical bi-pinconnectors; a rigid support structure enclosed within the elongatetubular housing and having a planar portion having a first surfaceextending within the elongate tubular housing between the first end andthe second end, the rigid support structure having spaced-apartsidewalls extending away from the first surface and extending within theelongate tubular housing between the first end and the second end, atleast a portion of the sidewalls in contact with an interior surface ofthe elongate tubular housing; a plurality of light emitting diodessupported by a second surface of the planar portion opposite to thefirst surface and between the first end and the second end, theplurality of light emitting diodes arranged to emit light through theelongate tubular housing; and a power supply circuit including a pulsewidth modulator and at least two current limiting elements, the at leasttwo current limiting elements configured to receive power from the pulsewidth modulator and to transmit power to at least some of the pluralityof light emitting diodes wherein the support structure divides theelongate tubular housing into a first space in which the plurality oflight emitting diodes are housed and a second space defined by theplanar portion, the sidewalls and the interior surface of the elongatetubular housing.
 19. The replacement light tube of claim 18 wherein atleast a portion of the power supply circuit is packaged inside theelongate tubular housing.
 20. The replacement light tube of claim 18wherein at least one of the at least two current limiting elementscomprises an inductive element electrically coupled between the pulsewidth modulator and the at least some of the plurality of light emittingdiodes.
 21. The replacement light tube of claim 18 wherein the at leasttwo current limiting elements and the pulse width modulator are packagedoutside of the elongate tubular housing.
 22. The replacement light tubeof claim 18 wherein the power supply circuit includes a rectifierconfigured to receive alternating current (AC) input and to providedirect current (DC) output.
 23. A replacement light tube for replacing afluorescent light tube, comprising: an elongate tubular housing having afirst end and a second end; a first end cap and a second end capdisposed on the first end and the second end, respectively, eachconfigured to fit with a socket for the fluorescent light tube; a rigidsupport structure enclosed within the elongate tubular housing andhaving a planar portion having a first surface extending within theelongate tubular housing between the first end and the second end, therigid support structure having spaced-apart sidewalls extending awayfrom the first surface and extending within the elongate tubular housingbetween the first end and the second end, at least a portion of thesidewalls in contact with an interior surface of the elongate tubularhousing; a plurality of light emitting diodes supported by the firstsurface between the first end and the second end, the plurality of lightemitting diodes arranged to emit light through the elongate tubularhousing; and a power supply circuit including a pulse width modulatorand at least two current limiting elements, the at least two currentlimiting elements configured to receive power from the pulse widthmodulator and to transmit power to at least some of the plurality oflight emitting diodes, the power supply circuit being packaged within atleast one of the end caps.
 24. The replacement light tube of claim 23wherein the rigid support structure has a substantially uniformcross-section along a length of the elongate tubular housing.
 25. Thereplacement light tube of claim 23 wherein the first surface extends ina plane within the elongate tubular housing and wherein the sidewallsextend sufficiently far from the plane such that light emitted by thelight emitting diodes is confined to an included angle of less than 180°by the sidewalls.
 26. A replacement light tube for replacing afluorescent light tube, comprising: an elongate tubular housing having afirst end and a second end; a first end cap and a second end capdisposed on the first end and the second end, respectively, eachconfigured to fit with a socket for the fluorescent light tube; a rigidsupport structure enclosed within the elongate tubular housing andhaving a planar portion having a first surface extending within theelongate tubular housing between the first end and the second end, therigid support structure having spaced-apart sidewalls extending awayfrom the planar portion and extending within the elongate tubularhousing between the first end and the second end, at least a portion ofthe sidewalls in contact with an interior surface of the elongatetubular housing, the planar portion being integral with the sidewalls; aplurality of light emitting diodes supported by the first surfacebetween the first end and the second end, the plurality of lightemitting diodes arranged to emit light through the elongate tubularhousing; and a power supply circuit including a pulse width modulatorand at least two current limiting elements, the at least two currentlimiting elements configured to receive power from the pulse widthmodulator and to transmit power to at least some of the plurality oflight emitting diodes.
 27. The replacement light tube of claim 26wherein the sidewalls extend from the first surface.
 28. The replacementlight tube of claim 26 wherein the sidewalls extend from a secondsurface of the planar portion, the second surface being opposite thefirst surface.
 29. The replacement light tube of claim 26 wherein aportion of the power supply circuit is external to the elongate tubularhousing and the first and second end caps.
 30. The replacement lighttube of claim 26 wherein at least a portion of the power supply circuitis packaged within the elongate tubular housing or at least one of theend caps.
 31. The replacement light tube of claim 26 wherein the pulsewidth modulator is configured to receive power from a power source andconfigured to regulate the intensity of light emitted from the pluralityof light emitting diodes by cyclically switching the received power onand off according to a varying duty cycle.